Field of the Invention
The invention lies in the automotive field. Specifically, the invention relates to a diagnostic device for monitoring an exhaust gas recirculation (EGR) system of an internal combustion engine. The system has an exhaust gas recirculation line which connects an exhaust gas tract to an intake tract. The opening cross section of the recirculation line is set by means of an exhaust gas recirculation valve, which is controlled by a control device. A pressure sensor disposed in the intake tract detects an intake pipe pressure.
Exhaust gas recirculation systems of internal combustion engines serve the purpose of reducing the NO.sub.x (nitrogen oxide) component in the exhaust gas. Since the exhaust gas of the internal combustion engine is an inert gas as to its essential constituents, it is possible by mixing exhaust gas with the intake combustion air to lower the peak combustion temperature and thus to reduce the emission of nitrogen oxides. The mass of the recirculated exhaust gas in the ratio to the sum of the mass of a fresh gas (intake air) and the mass of the recirculated exhaust gas is denoted below as the exhaust gas recirculation rate or EGR rate.
Given an exhaust gas recirculation rate of 10%, the nitrogen oxide emissions are already reduced by approximately 50%. The HC (hydrocarbon) and CO (carbon monoxide) emissions of the internal combustion engine cannot be significantly reduced by an exhaust gas recirculation system. The exhaust gas recirculation rate must be set as accurately as possible, since otherwise a rise in the HC and CO components in the exhaust gas results in the case of an excessively high exhaust gas recirculation rate.
Exhaust gas recirculation systems can, moreover, also be used to lower the fuel consumption. In the case of such exhaust gas recirculation systems, a significant reduction in the fuel consumption in the partial load operation of the internal combustion engine is achieved by means of a very high EGR rate (&gt;20%), since the throttle losses and the wall heat losses in the combustion chamber of the internal combustion engine are reduced.
A prior art diagnostic device is known from German patent application DE 42 16 044 A1. There, an exhaust gas recirculation system of an internal combustion engine is monitored. The exhaust gas recirculation system comprises an exhaust gas recirculation line which connects an exhaust gas tract to an intake tract, and whose opening cross section can be influenced by means of an exhaust gas recirculation valve. A control device for controlling the exhaust gas recirculation valve is also provided. It is known from DE 42 16 044 to design the diagnostic device in such a way that the temperature of the recirculated exhaust gas is measured in the exhaust gas recirculation line for the purpose of monitoring the required exhaust gas recirculation rate. However, that monitoring operation is not reliably possible in the specific case of a high exhaust gas recirculation rate, since the gradients of the temperature rise are very flat and it is impossible to eliminate disturbances owing to the environmental conditions such as, for example, the engine compartment temperature.
It is also known from DE 42 16 044 A1 to detect the change in the intake pipe pressure upon opening or closing of the exhaust gas recirculation valve, and thus to detect whether exhaust gas is flowing properly through the exhaust gas recirculation duct when the exhaust gas recirculation valve is opened. For that purpose, the exhaust gas recirculation valve is closed and subsequently opened in a prescribed operating state of the internal combustion engine such as, for example, in overrun conditions. Active intervention in the control of the exhaust gas recirculation valve is therefore necessary, and this leads to problems with emissions and the drivability of the motor vehicle.
The prior art diagnostic device is designed in such a way that it carries out a detection of engine cutout in prescribed operating states of the internal combustion engine. However, there is again in this case also an active intervention in the exhaust gas recirculation control.